Laminates, including decorative laminates, have been made for many years in a plural stack-up laminating operation. In this operation, a plurality of laminate stack-up sets, each set having two stack-ups containing a core layer plus at least one decorative layer, foil, or other surface layer, are placed between press plates with core layers facing each other. To prevent the stack-ups of each set from sticking to each other, release sheets are placed between the core layers of the stack-ups in the set, as described, for example, by Emily Jr. et al., in U.S. Pat. No. 3,050,434. As many as 6 stack-up sets, with associated press plates and release sheets, may be placed between heated platens to provide a laminate assembly or pack. In commercial operations, as many as 20 assemblies may be heat and pressure consolidated at each run of the laminating press, to produce, for example, 5 foot.times.12 foot decorative or metal clad laminates.
The standard release sheet for many years has been release treated glassine paper. Production of this paper requires a large commercial operation with many involved process steps, and the product is relatively expensive. Use of release treated glassine paper requires that it be handled as a separate type sheet, requiring separate cutting and handling operations. The glassine paper includes a release material, usually a chromium complex, such as stearato chromic chloride, and after separation of the laminates, must be completely, deep sanded off the core to which it bonds, so that the laminate will be glue adherable.
Recently, attempts have been made to produce in-house release sheets from standard Kraft paper core stock, to reduce cost, handling, and sanding operations. In Emily Jr. et al., U.S. Pat. No. 3,050,434, Kraft paper core sheet was impregnated with phenolic resin and then coated with a film of the salt of an alginic acid, such as sodium alginate, for use as a release sheet. After release, sanding off the alginate film was usually required. It was found however, that the alginate salt was mostly absorbed by the paper, with resultant release problems. Hagen, in U.S. Pat. No. 3,215,579, first sized the Kraft paper core sheet with an aqueous solution of a water soluble alkaline earth or alkaline earth metal salt, such as calcium chloride and the like, before impregnating with phenolic resin, and finally coating with a film of the salt of an alginic acid.
Even the Hagen triple operation seemed to present problems of salt absorption, release, and cost, and so, Jaisle et al., in U.S. Pat. No. 4,263,073, eliminated the phenolic resin completely, and used a particular type of paper web having a water absorption of at least 200 seconds. The use of this special paper caused the alginate salt to be retained on the surface fibers of the release sheet, with substantially no penetration or loss into the interior of the release sheet, thus functioning better as a release.
Jaisle et al. then went on, to further improve the release, in U.S. Pat. No. 4,243,461, by including either a triglyceride or a hydrolyzed or non-hydrolyzed lecithin with the alginate salt used to coat the resin-free, highly surface absorbent, special 200 second series paper. With both Emily Jr. and Hagen, alginate penetration of the release sheet seemed to pose a problem, so that substantial sanding was required. Jaisle et al., in addition to using specialized paper, would also require complete sanding because the paper was not resin impregnated with phenolic resin, as was the core. What is still needed is a simple, inexpensive, in-house method of making release sheets, and releasing laminates from one another, requiring minimal sanding.